This project investigates the effect of neurological and pharmacological interventions on the incidence and structure of squirrel monkey vocalizations emitted during standardized conditions in the laboratory. Current work focuses on mechanisms underlying expression of the isolation call, a characteristic vocalization used for reestablishing contact between an infant and its caregiver, and between adult social partners. Previous reports have documented the critical role of mediofrontal limbic cortex in expression of spontaneous isolation calls in squirrel monkeys, and the ability of chemicals selective for specific receptor subtypes (opiatergic mu receptor; alpha-2 adrenergic) to modulate the rate of production of this vocalization. New findings this year are: (1) subadult rhesus macaques with bilateral ablations of the amygdala continue to emit the species-typical isolatoin call, but the acoustic structure of the calls of these monkeys is abnormal, relative to age-matched controls, in exhibiting a less pronounced peak frequency in the time course of the fundamental, suggestive of reduced affective tone; (2) dose-response relations for isolation call production have been determined for the opiate antoagonist nalone over a dose range of 0.4-3.2 mg/kg, administered i.m. in adult squirrel monkeys; (3) the combined administration of naloxone and yohimbine (a specific alpha-2 adrenergic receptor antogonist) does not enhance isolation call rate, but results in a significant increase in the twitter call in separated squirrel monkeys, suggesting that under certain conditions, drugs with known anxiogenic properties may enhance behavior associated with positive affect; (4) pentobarbital sodium in sedating doses is effective in greatly enhancing production of the isolation call in adult squirrel monkeys; or (5) studies of the pharmacology of another squirrel monkey vocalization, the alarm call, suggest a role of the benzodiazepine receptor in the neurochemical control of the alarm reaction.